Heater control



May 30, 1950 A. R. COLLINS HEATER CONTROL Filed Feb. 27. 1946 atentedltil 195a fllrihur ltt. Collins, Parts Midge, lill, assignor toStewart-Warner worporation, tlhicago, till, a corporation oil Virginiaapplication ltehruary 2t, wit, serial l lo. stucco it illaims. Wt.Eddth} l The present invention relates to heater controls and is, moreparticularly, directed to a control system adapted for use with a heateroi the general type disclosed in the patents to 'iheodore it. Korsgren2,482,552, issued Septernher 2d, liittl,

and Frank A. Ryder 2,502,345, issued March 28. I

195a. It will be appreciated, however, that at though the control systemof the present inven tion is particularly adapted for use with heatersoi this type, it is not limited to such use hut is or more generalapplication.

"it is an object of the present invention to pro vide a control systemfor a heater which will modulate the output of the heater according todemand conditions.

An additional object of the present invention is to provide a novelheater control circuit and mechanism which operates the heater at a lowrate of heat output unless greater heat output is necessary to satisfythe prevailing conditions.

Yet another object of the present invention is to provide an improvedheater control arrangement which prevents the heater from cycling on andon excepting under extremely mild weather conditions.

Still another object of the present invention is to accomplish all ofthe above with a mechanism or low cost and which is safe and reliable inoperation.

Other objects and advantages will become apparent from the followingdescription of a preferred embodiment of my invention which is 11lustrated in the accompanying drawings.

lnthe drawings, in which similar characters of reference are used toindicate similar parts in tooth the figures:

Fig. l is a diagrammatic representation oi a heater oi the typedisclosed in the previously re ierred to copending applications; whileFig. 2 is a diagrammatic representation oi the control system for usetherewith which illustrates the present invention, including bothmechanical and electrical components.

As explained in the previously referred to co pending applications, theheating system with which the control arrangement of the present invention is particularly adapted for use is comprised of a variable speedelectric motor it which drives a ventilating air hlower it and acombustion air hlower it. The combustion air hlower receives its airthrough a duct it which leads preferably to the outside of the buildingto be heated. The ventilating air blower it receives its air through anopening it which map he in the space to he heated. Air from these twohill hit

tth

- at sources, that is, the ventilating air blower ill and the combustionair hlower it, is passed into the heater proper Ht where the ventilatingair circalates over a heat exchanger it and leaves the heater at anappropriate opening so as to pass into the space to be heated. Withinthe heater, the combustion air is mixed with a gaseous fuel deliveredthrough a pipe it and this mixture is ignited by a spar-it plug it whichreceives en ergy irorn an ignition transformer hi. Products atcommotion, after passing through the heat exchanger it, are exhausted tothe atmosphere remote from the space to he heated through the exhaustpipe liy means or" a modulating control valve lit, the pressure on thefuel passing through the pipe it to the heater varies depending upon therate oi combustion air flow from the blower i This is accomplished bymeans of a static pressure tap t l connected to the combustion airconduit and a low pressure tap 36 connected to the throat oi? a venturiit located in the same conduit. It is apparent that these connectionsare such that the greater the rate of flow of comhustion air, thegreater the pressure difierential existing between the taps it and 3B.This pressure difierence is communicated to the valve ti and as isexplained in the previously referred to copending application of FrankA. Ryder, this pressure acts to vary the pressure upon the fuel suppliedto the heater, such that the mixture ratio in the heater is keptcomparatively constant regardless of the rate oi flow of combustion air.Thus, the heat output of the heater depends upon the voltage across thevariable speed motor ill. The higher this voltage, the higher the speedof rotation of the motor and blowers, and thus the greater the heatoutput. The converse is, oi course. also true.

The control arrangement particularly adapted, but not limited, to usewith a heater of this type and which forms the subject matter oi thepresent invention comprises a heat motor it made up of a cylindricalcontainer it having an inturned annular flange til at its open endsealed to the open endoi a corrugated metal bellows at. at its upper oropposite end, the container wall is spaced from the closed end as of thebellows sutflciently to provide room for an electric heating element W.The space within the container ti and around the bellows M is filledwith a liquid which expands when heated. Acetone has been foundsatisfactory for this purpose.

.dt its upper end it, the inside surf-ace of the hollows is connected toa linlrti which extends out through the open end thereof and isconnected by means oi a pivot pin ht with an intermediate point of anarm 58 pivoted at one nd to a stationary portion of the structure at 68.The opposite end of this arm is prowlcled with a yoke 60. The spacingfrom the yoke 69 to the pin 54 is great as compared to the spacing be-=tween the points 54 and to, so that the yoke moves in the same directionas the link. 52 but to a considerably greater extent.

The yok 60 straddles a pin '52 at the end of a variable resistor sliderarm bt pivoted at a point 66 adjacent the pin 52. The opposite end orthe arm 64 is equipped with a slider tit which is in contact with a coiltype resistance element 70, one end of which is connected by way of awire 12 to the blower motor it, the op osite side of the motor beingconnected r supply by a lead it. As the slider a high resistance end ofthe elemen end in the illustration, slider operates to open a switch t:connected between the slider arr other side of the power the resistanceelement it'll causes the within the container to expand, th the upperend of the bellows 3161 This movement, operating through swings the arm56 downwardly arou point 58 so that the yolre til, by us pin 62, swingsthe slider t8 toward the motor, thereby decreasing the the motor circuitand increasing the s blowers i2 and it. Whenever the electric heater 50is tie-energized a sufficient length or time to permit the bellows 56 toelongate a predetermined amount, the slider 68 will be swung almostcompletely to the high resistance end of the resistor 10, at which pointthe motor it is operating at the lowest speed consistent with efucientheater combustion. If under these conditions the heat output of theheating system is still more than is required, the slider -58 will movea slight additional amount, whereupon the projection it opens the switchit, thus interrupting the motor circuit so as to discontinue heateroperation until such time as heat is required.

The controller for the electric heating element 50 of the heat motor itcomprises a bridge circuit made up of fixed resistors A and B which maybe identical and are so connected that one end of each is common, asindicated at this end being connected to one end of the heating ele mentEll by a wire 82. The opposite end to of the resistor A is connected tothe main line through a rectifier be which may be of th well knowncopper oxide type and which permits electric energy to flow from theline toward the resistance element A but not in the opposite direction.

Similarly, the opposite end to of resistor B is connected to the otherside of the power supply by way of a line 86. From the terminal til ofresistor B, the line 86 also leads to one end of a resistance elementD1, the other side of this latter resistor being connected by a wire toone end of a fourth resistance element Dz. At its opposite end, resistorD2 is connected to a wire 94 which leads to a variable resistor Cconnected on its other side to a line 95 leading to the power supply end=88 of the resistor A. The electric heater 50, previously mentioned asbeing connected on one side to the lead 82, is. connected at itsopposite end by a wire Qt to a second rectiher 98, while from theopposite side or the rectiresistance.

her a line I00 leads to a point [0| upon the wire 94 which connectsresistors D: and 0.

Of these resistors, A and B are of fixed value and have a. lowtemperature coemcient of resistance. In other words, changes intemperature have littl or no appreciable effect upon their Also, thevariable resistor C has a. low temperature coefllcient of resistivity.0n the other hand, the resistors D1 and D: have a high temperaturecoeilicient of resistance, such that an increase in the temperature ofthese units increases their resistanc considerably. The resistanceelement D1 is located within a cas M2 at some point along the inlet airduct iii. The case is equipped withan air inlet tube I04 which extendsinto the air duct i6 and has its open end facing upstream. A second tubeH16 leads from the case ltt and extends into the duct 16 so as to facedownstream therein. Thus, whenever air is flowing through the duct ii, aportion thereof will be diverted into the box 402 so as to circulatearound the resistor D1 and then return to the duct. The resistanceelement D1 is, therefore, maintained at the temperature of the airoutside of the building to be heated with the result that its resistanceincreases whenever the outside air temperature increases. The resistorD2 is positioned within a case i138 located within the room to be heatedso that it s maintained at substantially the temperature of the airtherein.

Inspection of the circuit just described will disclose that theresistors A, B, and C form three sides of a bridge while the resistorsD1 and D: in series form the fourth side. Inasmuch as the resistance ofA and B is constant, it follows that whenever the resistance of D1 andD2 together is less than the resistance 6, the bridge will be unbalancedso as to cause current to flow through the heating element 50. On theother hand, if the resistors D1 and D2 together are at a high enoughtemperature so that their resistance is greater than the resistor C, nocurrent will flow through the heating element 50 because the rectifier38 prevents current transmission in the reverse direction.

The additional rectifier 84 is used because the circuit is shown asconnected to an alternating current source. If a direct current sourceis used, this rectifier 8t is unnecessary.

When the system is in use, the variable resistor E, which may be locatedwithin the same case I08 as the resistor D2, is adjusted to thetemperature desired. If the temperature setting called for is such thatthe resistance of the portion of the resistor C remaining in circuit isless than the resistors D1 and D2 together, no current will flow throughthe heating element 59. Since the bellows 48 is not heated, it will droptoward room temperature and will expand so as to swing the slider 68downwardly until the switch '56 has been opened. The heater is,therefore, in the off position.

If, under the above conditions, the temperature within the room drops,or if the temperature outside the space to be heated decreases, theresistance of the elements 102 or D1, respectively, will be decreased.Whenever the decrease in the resistance of either becomes sufficient sothat the resistance of the two in series is less than that of theresistor C, the bridge becomes unbalanced I and current is caused toflow through the heating element 50, thereby raising the temperature ofthe liquid filling within the case 42 so as to 5 close the switch I8 andstart the heater. If the temperature of the room is well below adesirable level, or if the temperature outside has dropped considerably,it is apparent that the resistance of the elements D2 or D1, or both,will drop sufficiently so that the greater bridge unbalance will .causea greater heating eifect in the element 50,

thereby causing the slider 68 to move so as to take still moreresistance out of the motor circuit and increase the heater output stillmore.

From the above it is apparent that the bridge circuit always operates toprovide heat whenever the temperature of the room is below asatisfactory level and that as the temperature of the room approachesthe satisfactory level, the heater is gradually modulated so as toreduce the heat 1 output. It is also apparent that this effect isinfluenced by the anticipating unit D1 which is located so that it cansense the temperature of the outside air. Thus, any sudden drop or risein the outside air temperature causes the heater to produce more or lessheat, respectively, even though the temperature within the space to beheated has not changed. This permits the heater to increase the heatoutput to the room so as to keep up with the increased heat loss fromthe room to the outside.

If it is desired that the room controller unit D: have a more pronoucedeflect in influencing heater operation than the unit D1, this can easilybe accomplished. For instance, merely,by decreasing'the resistance ofthe unit D1 relative to D2, so that changes in the temperature of theunit I): will have a greater eflect upon the total resistance in thatleg of the bridge than will changes in the temperature of the resistorD1,

, the desired result is achieved.

, sistor comprised of the coil in and slider 68. it

will be appreciated that, if desired, this unit may comprise a variablereactance of the inductive type or may be a variable transformer.

Having described my invention. what I claim as new and useful and desireto secure by Letters Patent of the United States is:

1. In a device of the type described, a motor speed controller, a heatresponsive device con hosted for operation of said controller, anelectric heating element for operating said heat responsive device, afirst resistor having a high temperature coeflicient of resistance andarranged to be sensitive to the temperature of the space to be heated, asecond resistor having ,a high temperature coeiiicient of resistancearranged to be sensitive to the temperature of the ambient air, avariable resistor having a low temperature coemcient of resistance, saidresistors being electrically connected so that both of said resistorshaving a high temperature coeflicient of resistance form one leg of abridge circuit, said variable resistor forming a second leg of saidbridge circuit, other resistor; having a low temperature coeiilcient ofresistance forming the other legs of said bridge circuit. circuit meansconnecting said bridge to a power source and a bridging wire for saidbridge containing said heating element, said bridging wire including arectifier for preventing energization of said heating element by saidbridge when said bridge is unbalanced in one direction but for causingcontinuous energization of said heating element when said bridge isunbalanced in the other direction.

2. In a device of the type described, a variable reactor. an electricheating element, means responsive to the heat output of said heatingelement for operating said reactor, a pair of resistors each having ahigh temperature coeflicient of resistance, said resistors beingconnected togather and forming one leg of a bridge circuit and beingarranged to be sensitive to different temperature conditions, a variableresistor'having a low temperature coefficient of resistance forminganother leg of said bridge, other circuit elements forming the other legof said bridge, circuit means connecting said bridge to. a power sourceand a bridging wire for said bridge containing said heating element,said bridging wire including a rectifier for preventing energization ofsaid heating element by said bridge when said bridge is unbalanced inone direction but for causing continuous energization of said heatingelement when said bridge is unbalanced in the other direction.

3. In a device of the type described, a variable reactor, an electricheating element responsive to an electric current for operating saidreactor, a bridge circuit for energizing said current responsive meanswhen said bridge is unbalanced, said bridge including temperatureresponsive resistors arranged to be sensitive to the temperature of thespace to be heated and of the ambient air and a bridging wire containingsaid electric heating element. and means to prevent current flowing fromsaid bridge to said electric heating element when said bridge isunbalanced in one direction. I

ARTHUR R. COLLINS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

